The general use of the term carcinoid for the classification of neuroendocrine tumors has become increasingly difficult during recent years. First, its definition and prognosis varies from site to site. Second, its traditional definition does not cover the whole morphological and biological spectrum of neuroendocrine tumors known today. We therefore propose new classifications of the neuroendocrine tumors of the lung, pancreas, stomach, duodenum, jejunum-ileum, appendix and colorectum. These classifications use a common framework and attempt to consider the morphological, functional and biological features of these tumors.